1. Field of the Invention
The present invention relates to an optically anisotropic film used for liquid crystal display apparatuses, nonlinear optical devices and the like, and a liquid crystal display apparatus using said film.
2. Description of the Related Art
A retarder obtained by uniaxially stretching a transparent thermoplastic polymer film, is used as an optical compensation film (hereinafter, sometimes referred to as color compensation film) for compensating coloration of liquid crystal display devices, particularly, super twisted nematic (hereinafter, sometimes referred to as STN) liquid crystal display devices and electrically controlled birefringence (hereinafter, sometimes referred to as ECB) liquid crystal display devices (hereinafter, sometimes referred to as LCDs) to improve their display qualities. Liquid crystal display devices using the retarder have the advantages of being light in weight, thin and inexpensive.
However, the STN LCD has the disadvantage of slow response time. To make the response time faster, it is effective to decrease the cell gap by using liquid crystals having a large birefringence .DELTA. n. Liquid crystals having a large .DELTA. n show, generally, a high wavelength dispersion of the birefringence, and it is known that the retardation of the retarder used with the liquid crystal cell also should have a high wavelength dispersion in order to compensate the high wavelength dispersion of .DELTA. n of the liquid crystals. Use of a retarder having a high wavelength dispersion of retardation is effective.
A liquid crystal display apparatus having high contrast is not obtained when a fast-response liquid crystal cell, i.e. a liquid crystal cell comprising a liquid crystal having a large .DELTA. n is used in combination with a retarder having a conventional wavelength dispersion of retardation. Hence, a retarder having a high wavelength dispersion characteristic corresponding to the wavelength dispersion characteristic of birefringence of the liquid crystal material used for fast response STN liquid crystal cells has been desired.
As a means for improving the wavelength dispersion of a retarder, JP-A-5-107413 discloses a retarder using polysulfone (hereinafter, sometimes referred to as PSf) having a high wavelength dispersion. Likewise, JP-A-6-174923 discloses the use of polyarylate having a high wavelength dispersion. However, polysulfone and polyarylate are difficult to process since their glass-transition temperatures are high, and this causes manufacturing problems.
Moreover, since the wavelength dispersion characteristic of retardation differs depending on the liquid crystal material used for the liquid crystal cell, it is sometimes necessary to control the wavelength dispersion characteristic of the retarder. JP-A-5-27119 and JP-A-6-130227 show that the wavelength dispersion can be controlled by a combination of optically anisotropic substances having different wavelength dispersion values.
JP-T-4-500284 shows a retarder using a side-chain liquid crystal polymer having a linear or cyclic main chain and having a temperature dependence and a wavelength dispersion the same as those of the liquid crystal molecules used for the liquid crystal cell. This prior art discloses that a liquid crystal polymer film is formed on the orientation film and that an oriented liquid crystal polymer film is obtained by applying a field such as an electric field or a magnetic field to the liquid crystal polymer film. However, since the liquid crystal polymer has a large refractive index anisotropy regardless of whether it is of main-chain type or of side-chain type, it is difficult to manufacture a large-area film of required thickness accuracy by using such a liquid crystal polymer.
JP-A-5-257013 describes a retarder having a wavelength dispersion of refractive index anisotropy the same as that of the liquid crystal molecules used for the liquid crystal cell by dispersing liquid crystal molecules into a polymer film and stretching them together with the polymer film. However, this prior art does not specifically describe a preferred wavelength dispersion characteristic, the physical properties and the structure of the liquid crystal and to what extent the liquid crystal should be dispersed.
JP-A-7-13683 shows in its 6 th embodiment an example where the wavelength dispersion is improved by blending polyvinyl chloride with a liquid crystal compound. However, a substance such as polyvinyl chloride having a low softening point can not be used in a high-temperature environment since deformation and deterioration in optical characteristics occur.
JP-A-7-13023 discloses a method for adjusting the wavelength dispersion by adding a plasticizer to polycarbonate or polyarylate. However, this prior art does not describe the addition of a dye which absorbs ultraviolet light.